The show's team completely covered a last-gen Ford Taurus with modelers clay and figured out that it would achieve about 26 mpg at a constant 65 mph. They then went about adding over 1,000 dimples to the car's exterior. To keep the experiment consistent, all 1,082 dimples removed from the clay exterior were put in a box and set in the back seat so that the car would weigh exactly the same as before dimpling. The theory is that, like a golf ball, the dimples would reduce the car's drag through the air, thus allowing it to travel the same distance at the same speed using less fuel. The result? Over 29 mpg.

Higher mass - It takes more energy to get up to 60mph
Lower mass - It takes less energy to get up to 60mph

Their tests were conducted after already at 60mph.

Apply Newton's First law, not Einstein's E=MC^2 theory.

Regardless of weight, once at 60mph the fuel consumption stays the same... weight doesn't affect air resistance, only marginally tire friction. The greater the mass, the greater the inertia, therefore the less impact tire friction will have.